Andreas F. M. Kilbinger

Surface-controlled self-assembly of chiral sexithiophenes

We report on the self-assembly of two enantiomeric sexithiophenes in solution and on surfaces. Circular dichromism of aggregated sexithiophenes and drop-cast films reveals, as expected, mirror image spectra for both enantiomers. The aggregation in thin deposits from sexithiophenes molecularly dispersed in a solution on different types of substrates was investigated by atomic force microscopy (AFM). On graphite, one-dimensional objects (nanowires) are formed while on mica platelets are generated. Remarkably, we found that both enantiomers form left-handed helices on silicon. This observation depends on the hydrophilicity of the silicon. Furthermore, the achiral sexithiophene did not form helical aggregates suggesting that the stereocenter is required to obtain chirality in the fibers.

End Capping Ring-Opening Olefin Metathesis Polymerization Polymers with Vinyl Lactones

The selective placement of a functional group at the chain end of a ring-opening metathesis polymer using ruthenium carbene initiators has been a significant limitation. Here we demonstrate a highly effective and facile end-capping technique for ROMP with living ruthenium carbene chain ends using single-turnover olefin metathesis substrates. Vinylene carbonate and 3H-furanone are introduced as functionalization and termination agents for the ruthenium-initiated ring-opening metathesis polymerization. This leads directly to the formation of functional polymer end groups without further chemical transformation steps. Aldehyde and carboxylic acid end groups can be introduced by this new method which involves the decomposition of acyl carbenes to ruthenium carbides. The high degrees of chain-end functionality obtained are supported by (1)H NMR spectroscopy, MALDI-ToF mass spectrometry, and end-group derivatization.

Ultrathin organic films for field effect transistors

Organic field effect transistors FET have been fabricated with active semiconducting organic thin films that are only a few monolayers thick. The motivation of this study has been to establish a direct correlation between transistor performance and the polymer microstructure in the ultrathin accumulation layer of the transistor. Monolayer thick films of a block copolymer and several model oligomers consisting of a rigid conjugated sexithiophene (6T) block and a flexible polyethyleneoxide (PEO) block have been deposited onto the surface of e.g. SiO2 gate dielectrics functionalized with a self-assembled monolayer. Block copolymer phase behavior and surface morphology has been studied as a function of chain length, solvent and film thickness. Operational transistors have been demonstrated with film thicknesses of only one or two monolayers. Typical device characteristics show a carrier mobility on the order of 10-2 - 10-3 cm2/Vs and ON-OFF current ratio of 103 - 105. Film microstructure, orientation of micro-crystallites and film thickness have been studied by atomic force microscopy (AFM), UV-Vis absorption spectroscopy and X-ray diffraction.

Polyethylenimine Is a Strong Inhibitor of Human Papillomavirus and Cytomegalovirus Infection

ABSTRACT Polyethylenimines are cationic polymers with potential as delivery vectors in gene therapy and with proven antimicrobial activity. However, the antiviral activity of polyethylenimines has not been addressed in detail thus far. We have studied the inhibitory effects of a linear 25-kDa polyethylenimine on infections with human papillomaviruses and human cytomegaloviruses. Preincubation of cells with polyethylenimine blocked primary attachment of both viruses to cells, resulting in a significant reduction of infection. In addition, the dissemination of human cytomegalovirus in culture cells was efficiently reduced by recurrent administration of polyethylenimine. Polyethylenimine concentrations required for inhibition of human papillomavirus and cytomegalovirus did not cause any cytotoxic effects. Polyethylenimines and their derivatives may thus be attractive molecules for the development of antiviral microbicides.

Ab-initio crystal structure analysis and refinement approaches of oligo p-benzamides based on electron diffraction data

Ab-initio crystal structure analysis of organic materials from electron diffraction data is presented. The data were collected using the automated electron diffraction tomography (ADT) technique. The structure solution and refinement route is first validated on the basis of the known crystal structure of tri-p-benzamide. The same procedure is then applied to solve the previously unknown crystal structure of tetra-p-benzamide. In the crystal structure of tetra-p-benzamide, an unusual hydrogen-bonding scheme is realised; the hydrogen-bonding scheme is, however, in perfect agreement with solid-state NMR data.

Solution processable alternating oligothiophene-PEO-block-co-polymers: synthesis and evidence for solvent dependent aggregation

A new route to oligothiophene–PEO-block-co-polymers has been developed, in which well-defined α-oligothiophene blocks (from bithiophene to sexithiophene) alternate with poly(ethylene oxide) blocks. These materials show high solubility in common organic solvents. UV/visible and fluorescence studies in solution indicate that the oligothiophene segments are molecularly dissolved in good solvents like chloroform. Aggregation of the oligothiophenes occurs in dioxane–water mixtures, which is manifest by shifts of the UV/visible absorption maxima towards the blue and quenching of the fluorescence. An oligothiophene length of three thiophenes (terthiophene) is necessary to observe this aggregation phenomenon.

Catalytic living ring-opening metathesis polymerization

In living ring-opening metathesis polymerization (ROMP), a transition-metal–carbene complex polymerizes ring-strained olefins with very good control of the molecular weight of the resulting polymers. Because one molecule of the initiator is required for each polymer chain, however, this type of polymerization is expensive for widespread use. We have now designed a chain-transfer agent (CTA) capable of reducing the required amount of metal complex while still maintaining full control over the living polymerization process. This new method introduces a degenerative transfer process to ROMP. We demonstrate that substituted cyclohexene rings are good CTAs, and thereby preserve the ‘living’ character of the polymerization using catalytic quantities of the metal complex. The resulting polymers show characteristics of a living polymerization, namely narrow molecular-weight distribution, controlled molecular weights and block copolymer formation. This new technique provides access to well-defined polymers for industrial, biomedical and academic use at a fraction of the current costs and significantly reduced levels of residual ruthenium catalyst. Ring-opening metathesis polymerization (ROMP) offers good control over dispersity, but the requirement of one initiator per chain can be expensive and problematic for purification. Now, a reversible cyclohexenyl-containing chain-transfer agent is described, thus allowing a catalytic living ROMP process to produce narrow dispersity polymers and block copolymers.

Anharmonic Magnetic Deformation of Self-Assembled Molecular Nanocapsules

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bolaamphiphilic sexithiophene molecules. At low fields the deformation--measured through linear birefringence-scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction [W. Helfrich, Phys. Lett. A 43, 409 (1973)10.1016/0375-9601(73)90396-4], and permit the determination of the bending rigidity of the capsules as (2.6+/-0.8) x 10(-21) J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behavior, and allows discussion of the formation and stability of nanocapsules in solution.

Rod–coil copolymers from oligo(p-benzamide) foldamers

Self assembling rod–coil copolymers were synthesized in which oligo(p-benzamide) rods up to the octamer were prepared via iterative solution synthesis employing the acid labile 2,4-dimethoxybenzyl amide protective group.

H-bonding schemes of di- and tri-p-benzamides assessed by a combination of electron diffraction, X-ray powder diffraction and solid-state NMR

The crystal structures of di- and tri-p-benzamides are solved by a combination of single crystal, electron and powder X-ray diffraction. Different hydrogen-bonding schemes observed in the two structures are described and classified. The hydrogen-bonding networks are correlated to complementary data obtained from multinuclear solid-state NMR.